The present invention relates to an edge-rounding method and an apparatus therefor, which are mainly suited for use in the manufacture of a floppy disc head, i.e., a magnetic head for recording and/or reproducing information on a floppy disc.
Referring to FIG. 1 of the accompanying drawings, the floppy disc head A (workpiece to be edge-rounded) is a generally rectangular cubic body made of a fragile material such as ceramic. A peripheral edge 2 on one of the opposite major surfaces (i.e., a to-be-ground surface 1) thereof is so chamfered as to avoid any possible damage to the floppy disc with which it is in sliding contact. The transition portion between a flat area 3 in the major surface or to-be-ground surface 1 and a rounded corner 2a is required to be smoothed with high preciseness.
FIG. 2 illustrates the cross-sectional profile of the head A taken in a direction parallel to the short axis Y lying on the to-be-ground surface 1. As shown, in order for the head A to function optimally, the standards stipulate that the smoothness of the transition area in the direction of the short axis Y must be within a tolerance of a/b=0.003 mm/0.020 mm. When this is expressed in terms of the angle .theta. of inclination of a curved surface of the rounded corner 2a at the transition area, the inclination angle will be 0.86.degree., and it it is therefore required that said curved surface be formed so as to have an inclination angle .theta. smaller than 0.86.degree..
A prior art edge-rounding machine capable of effecting a highly precise edge-rounding is illustrated in FIGS. 3 and 4.
In this prior art edge-rounding machine, the edge-rounding is carried out by the use of a grinding device 6 having an abrasive sheet 5 mounted on a rotary disc 18 by means of an elastic sheet 4, and a workpiece holder assembly 7 capable of imparting to the workpieces A rotary and pivotal motions and a biasing force acting in a direction parallel to the axis of rotation of the workpieces A.
As shown, the holder assembly 7 is provided with three holders 8 for the support of the respective workpieces A with their to-be-ground surfaces oriented downwards, such that the three workpieces A can be simultaneously processed. As best shown in FIG. 4, each of the holders 8 is capable of being rotated by a motor 9 about the axis P' of rotation concentric with the longitudinal axis thereof so as to impart rotary motion to the respective workpiece A. In addition, each holder 8 is supported by a holder support 10 for movement up and down and is normally urged downwards by a spring 11. As a result, each of the workpieces A is urged in a direction parallel to the axis P' of rotation with the peripheral edge 2 of the to-be-ground surface 1 elastically loaded to contact the abrasive sheet 5 positioned therebelow. The holder support 10 is fitted to a free end of a rocking arm 12 and is capable of pivoting about an axis Q' within a predetermined angle .alpha.' incident to the rocking motion of the rocking arm 12. Therefore, the holder 8 and, hence, the associated workpiece A, is given a pivotal motion about the axis Q' in FIGS. 3 and 4. In FIGS. 3 and 4, reference numeral 13 designates a cranking mechanism for rocking the rocking arm 12, reference numeral 14 designates a piston-cylinder device for moving the workpiece holder assembly 7 up and down relative to a machine framework, and reference numeral 16 designates a motor for rotating the rotary disc 18 of the grinding device 6.
According to the prior art, and as hereinabove described, the edge-rounding is performed by causing the peripheral edge 2 of the to-be-ground surface 1 of each workpiece A to contact under pressure the abrasive sheet 5 of the grinding device 6 while the respective workpiece A is rotated, pivoted and urged in the direction parallel to the axis P' of rotation. The range of the angle through which the respective workpiece A is pivoted lies on one side of a vertical line V' passing through the pivot axis Q' and perpendicular to the abrasive sheet 5, thereby avoiding the possibility of the flat area 3 of the workpiece A contacting the abrasive sheet 5, i.e., to avoid the formation of traces of grinding on the flat area 3 of the workpiece A. In addition, when the workpiece A is pivoted to a position closest to the vertical lines V' as shown by the phantom line in FIG. 4, the machining of the curved surface at the transition area between the flat area 3 and the rounded corner 2a is carried out. At this time, the elastic sheet 4 underlying the abrasive sheet 5 is elastically deformed to permit the edge-rounding to result in the inclination angle .theta. smaller than 0.86.degree..
However, it has been found that the prior art method described above has the following disadvantages.
(i) Initial deviations, and changes with time, of the elastic modulus, surface hardness and other factors of the elastic sheet 4 adversely affect the inclination angle .theta. and, therefore, the quality of the workpieces A which have been edge-rounded tends to be unstable. PA1 (ii) The standards for the shape of the curved surface of the edge-rounded corner 2a vary with the type of machine in which the floppy disc head is employed. In order for workpieces A to be manufactured which have different shapes of edge-rounded corners 2a, a number of different kinds of elastic sheets 4 must be prepared, and in practice the various different elastic sheets must be chosen by trial and error to achieve the manufacture of floppy disc heads for the particular types of machines. In addition, complicated grinding conditions must be selected. PA1 (iii) Since according to the prior art described above the entire process from the coarse grinding to the final grinding is performed on the single abrasive sheet 5 on the rotary disc 18, not only is the grinding efficiency low, but also the manufacturing cost is high as a result of the consumption of the abrasive sheet, which in turn brings about an increased price of the resultant product. PA1 (iv) The prior art method employs a batch system wherein a grinding machine is employed for each process step of coarse grinding to final grinding. Therefore, each time the workpieces are transferred from one process step to another, the workpieces have to be removed from the grinding device and, therefore, not only can grinding with high preciseness be rather difficult, but also the productivity is very low.